1. Field of the Invention
The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
In order to prevent undesired movement of the particles, such as sedimentation, partitions between the two electrodes were proposed for dividing the space into smaller cells (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)). However, in the case of partition-type electrophoretic displays, difficulties were encountered in the formation of the partitions and the process of enclosing the suspension. Furthermore, it was also difficult to keep suspensions of different colors separate from each other in the partition-type electrophoretic display.
Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs (the channel or groove type). The filling and sealing of the electrophoretic fluid in the channels are accomplished by a batch-wise process. However, the problem of undesirable particle movement or sedimentation, particularly in the longitude direction, remains an issue.
Subsequently, attempts were made to enclose the electrophoretic dispersion in microcapsules. U.S. Pat. Nos. 5,961,804, 5,930,026, 6,017,584, 6,067,185, 6,262,706 and U.S. Patent Application Pub. No. 2002/0185378A1 published on Dec. 12, 2002 describe microcapsule-based electrophoretic displays. The microcapsule type display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric solvent and a suspension of charged pigment particles that visually contrast with the dielectric solvent. The diameter of the electrophoretic microcapsules is usually in the order of 101 to 102 μm and are typically prepared in an aqueous phase by a process such as simple or complex coacervation, interfacial polymerization or in-situ polymerization. Review of microencapsulation processes can be found in, for example, A. Kondo, “Microcapsule Processing and Technology”, Marcel Dekker, Inc., (1979); J. E. Vandegaer, ed., “Microencapsulation, Processes and Applications”, Plenum Press, New York, N.Y. (1974); and Gutcho, “Microcapsules and Microencapsulation Techniques”, Noyes Data Corp., Park Ridge, N.J. (1976), all of which are herein incorporated by reference. In the coacervation process, an oil/water emulsion is formed by dispersing the electrophoretic composition in an aqueous environment. One or more colloids are coacervated out of the aqueous phase and deposited as shells around the oily droplets through control of temperature, pH and/or relative concentrations to create the microcapsules. The interfacial polymerization approach relies on the presence of an oil-soluble monomer in the electrophoretic composition which is present as an emulsion in an aqueous phase. The monomers in the minute hydrophobic droplets react with a monomer introduced into the aqueous phase, polymerizing at the interface between the droplets and the surrounding aqueous medium and forming shells around the droplets. In the in-situ polymerization approach, the monomers that will form the microcapsule shell are present in the aqueous phase rather than within the dispersed-phase droplets.
After formation of the microcapsules containing the electrophoretic composition, the microcapsules may be printed or coated onto an electrode substrate by a method such as that used to deposit pressure-rupturable microcapsules onto a substrate to create a carbonless copy paper. The microcapsules may be immobilized within a transparent matrix or binder and sandwiched between two electrodes or substrates. Alternatively, a polymeric protection layer may be overcoated onto the microcapsule layer for applications such as the rewritable recording sheet as disclosed in, for example, U.S. Pat. No. 6,473,072 and U.S. Patent Application Pub. No. 2001/0055000A1 filed on Apr. 2, 2001.
The electrophoretic displays based on microcapsules prepared in an aqueous phase, however, suffer many drawbacks. For example, in order to stabilize the oil-in-water emulsion, a hydrophilic surfactant or protective colloid is needed. It, however, is often very difficult and costly to remove the unwanted hydrophilic additives from the capsule surface and the aqueous phase. Secondly, the shell formed at the capsule/water interface tends to be hydrophilic and softenable or plasticizable by moisture. Thirdly, strong charge controlling agents (CCAs) typically employed to enhance the electrophoretic mobility or switching rate of an electrophoretic display are often very difficult to be encapsulated in the dispersed phase. They tend to migrate to the water-shell interface or the aqueous phase during the encapsulation process, and as a result, are not effective in enhancing the switching rate. Further, in the capsule-based display, a binder is an essential ingredient of the capsule coating or printing composition. For microcapsules prepared from an aqueous phase, a water-based binder may be used at the expense of resistance to humidity in the final display prepared. Alternatively, the capsules may be dried, purified and redispersed into a non-aqueous binder. However, the drying, purification and redispersion steps are often of low yield and high cost because of irreversible flocculation or coagulation and undesirable rupture of the capsules. The above-mentioned problems associated with the capsules prepared in an aqueous phase result in electrophoretic displays of poor humidity resistance, low electrophoretic mobility or low switching rate and high cost of manufacturing.
Alternatively, capsules may be formed by dry processes similar to those employed to form the dispersed type of liquid crystal displays [See Drzaic in “Liquid Crystal Dispersions” (1995)]. For example, U.S. Pat. No. 5,930,026 discloses a process of forming a capsule type of electrophoretic coating on an electrode substrate by first emulsifying an electrophoretic composition in a UV curable resin, followed by UV curing to suspend and immobilize the emulsion droplets in the cured resin on the substrate to form a polymer-dispersed electrophoretic display. No oil-in-water emulsion is involved in this process. However, a very broad capsule size distribution with a significant amount of useless small capsules is often obtained. It is very difficult to control the capsule size distribution, and once the capsules are formed, there is no way to remove the unwanted capsules.
To prepare humidity resistant electrophoretic capsules that are compatible with strong CCAs, an encapsulation process such as interfacial or in-situ polymerization/crosslinking in a non-aqueous phase may be employed. However, it is very difficult to find a satisfactory immiscible solvent pair in which one is the dielectric solvent of the electrophoretic fluid phase and the other does not compete with the pigment particles in the dielectric solvent for CCAs or the protective colloid(s). Moreover, to achieve satisfactory physicomechanical properties, the capsule shell must be a good solvent barrier with a thickness sufficient to protect the capsules from being unintentionally ruptured during handling or coating. Therefore there has been a strong need for capsule-based display cells having improved physicomechanical properties and also a solvent resistant shell of acceptable thickness.